Patent GB-1039323-A (GB)
Title: Improvements in or relating to radar systems
IPC: G01S13/70
Filed: 1963-02-05
Abstract: 1,039,323. Radar. HONEYWELL Inc. Feb. 5, 1963 [Feb. 5, 1962], No. 4677/63. Heading H4D. Electronic distance measuring equipment comprises a transmitter to transmit energy pulses towards a reflecting object the distance of which is to be measured, a receiver for receiving and amplifying reflected pulses and producing a video pulse signal, each pulse having a sloping leading edge, a time-modulator circuit operated in synchronism with the transmitter and producing a periodic gating signal output that is controllable as regards its timing relative to the transmitted pulse, said gating signal consisting of pulses narrower than the video pulses to intersect the sloping leading edges of the video pulses, a gate circuit that receives the video pulse and the gating pulse as two inputs and passes the video pulse whilst it coincides with the gating pulse, and an integrating network that receives at its input the output of the gate circuit, together with a bias signal and delivers a signal representing the required distance, the bias signal balancing the gate circuit output that occurs when the gating pulse and video pulse intersect at a predetermined track point along the sloping leading edge, and the integrating network output being applied to control the time modulator circuit, such that said track point is continuously tracked. Pulse modulator 11 produces 1000 W. pulses of 10<SP>-9</SP> sec. in width at 40 kc/s. P.R.F. to drive a transmitter 12 to produce 100 W. pulses for transmission by antenna 13. The echo pulses are received by antenna 18, changed to an I.F. signal, amplified and detected in means 22, and the detected resultant pulses are amplified in video amplifier 31. A noise A.G.C. arrangement 142, constructed to ignore the steep echo pulses controls the noise amplification of amplifier 22, via A.G.C. integrator 86. The amplitude of the echo pulse at the output of the amplifier 31 is kept constant by a second A.G.C. circuit incorporated in arrangement 82. The transmitter 12 also produces synchronized timing pulses which are fed to a sawtooth generator 36, to produce a train of sawtooth waves, synchronized with the transmitted pulses, for feeding to one input of a voltage comparator 40. The sawtooth waves are compared with the output of a double integrator 54 to give an output pulse every time the two voltages are equal, for feeding to a gate generator 41, to produce a narrow gating pulse at terminal 46 and a second gating pulse, having its leading edge coincident with that of the narrow pulse, but having a width equal to that of the echo pulses. The two gating pulses are fed respectively to two coincidence gates 48 and 82, the other inputs of which are fed with the video signal from the amplifier 31. The narrow gating pulse tracks along the leading sloping edge of the echo pulse and passes a signal to double integrator 54 representative of delay of the narrow gating pulse with respect to the start of the echo pulse on foot of the leading edge. The output of the first integrator 53 represents the velocity of the target whilst the output of the second integrator 55, which is fed to comparator 40 represents the distance of the target. The narrow gating pulse is thus caused to track the foot of the leading edge of the echo pulse, with velocity and position memory facilities if loss of the echo pulse should occur. The entire echo pulse is passed through gate 82 and is used to control the amplification of I.F. amplifier 22 so that the amplitudes of the pulses are the same as the output of the video amplifier 31, irrespective of their initial amplitude. The output of gate 82 is also passed to a twostate (track and search) switching circuit 92 to hold it in its first (track) state. The circuit then produces a signal, energizing relay 71 to close contact 70, and a signal, energizing relay 77 via delay means 115 to close contact 76. The output from the second integrator 55, representing the altitude of the aircraft, can then be passed to an altitude indicating means via a smoothing circuit 73. The switching circuit 92 also produces in its first state a signal causing a recycle circuit 106 to periodically switch the switching circuit to its second (search) state to make sure that the altimeter is not locked on to a false target. The interval during which the altimeter is searching is so short that the chance of loss of the target during the interval, is negligible. When the echo pulse is lost, no signal appears at the output of gate 82 and the switching circuit 92 is switched to its second (search) state. In this state the switching circuit produces a signal on line 98 inhibiting integrator 53, and output 100 feeds a positive step voltage to the input of integrator 55 to produce a rising ramp voltage at the integrator output. This ramp voltage is passed to the comparator 40 to cause the narrow gating pulse to walk forward in the transmission period to search for an echo pulse. The ramp voltage is also fed to recycle multivibrator means 94, wherein, when the ramp reaches its maximum voltage a negative step voltage is produced by the multivibrator means 94 and is fed to the input of the integrator 55 to produce a falling ramp voltage at the integrator output. This ramp causes the narrow gating pulse to walk backward in the transmission period and repeat the search. When the falling ramp voltage reaches its minimum voltage at the stop recyclic input 120, the stop recycle means 95 produces a signal stopping the negative step voltage, allowing the positive step voltage from switching circuit 92 to control the integrator and restart the search. Also, in the search state, the signals energizing the relays 71 and 77 are removed but only after a delay to allow for the case when the loss of the echo signal is only momentary. The smoother circuit 73 contains a memory device and remembers the aircraft's altitude at the moment the contacts 70 open. Range switch 112 operates in three manually selectable modes, short range, long range and automatic. In the automatic mode, in the track state, if the altitude is within the short range but increasing, the output of the integrator 55 causes the recycle multivibrator 94 to pass a signal to the range switch 112, when the maximum of the short range is reached, causing the range switch to change to the long range mode. Appropriate signals are then fed to the sawtooth generator 36 and first integrator 53. Similarly the stop recycle means 95 passes a pulse to the range switch 112 when the aircraft's altitude decreases from the long range to the short range. In the search state an inhibiting signal at the range switch input 111, from the switching circuit 92 is removed and the range switch periodically changes the range from short to long and back again. A sensitivity control 63 comes into operation when the altitude decreases below a predetermined limit, to reduce the gain of the amplifier 22 to avoid overloading the receiver. The pulse width of the transmitted pulses correspond to about 30 ft., so that below this altitude the transmitted and echo pulses overlap. The A.G.C. circuit, however, reduces the receiver amplifications to make the effect of transmitter leakage small. In addition the separate transmitting and receiving antennae are mounted to minimize leakage effects. The transmitter/modulator circuit, Fig. 11 (not shown), includes a blocking oscillator 1030, comprising a parallel connected double triode 1031, tuned feedback transformer 1041 and P.R.F. varying potentiometer 1063, directly feeding the input of a resonant cavity 1071, which in turn produces an R.F. pulse for radiation by antenna 1077.